Non-reactive gypsum plaster mold and method of casting therein



United States atent O NON-REACT1VE GYPSUM PLASTER MOLD AND METHQD 6F tCAliiNC-l THEREIN William E. Nolan, Buffalo, N. Y, assignor to Aluminum Company of America, Pittsburgh, Pa, in corporation of Pennsylvania No Drawing. Application March 14, 1952., Serial No. 276,635

llll lClaims. (Cl. 22-2165) This invention relates to molds composed in large part of gypsum plaster, also known as plaster of Paris, which have been rendered substantially nonreactive toward easily oxidirible metals such as magnesium and magnesium base alloys. The invention is especially concerned with those molds that are permeable to the passage of gases from the mold cavity during the casting operation.

Non-porous molds of plaster of Paris have long been used for making ornamental articles of precious metals and more recently molds of this type have been used for the production of small precision castings of high melting point alloys. Molds of this type are relatively dense and do not permit the easy escape of gases when molten metal is introduced therein. This disadvantage has been overcome by the development of mold mixes and manufacturing techniques which provide a permeability substantially equivalent to that found in conventional sand molds and cores. With the advent of permeable plaster molds it became possible to use this type of mold more generally in foundry operations.

While many metals can be poured in plaster molds without encountering any reaction between the metal and the mold material it has been found that easily oxidizible metals, such as magnesium and magnesium base alloys, cannot be handled in the same manner as other metals. My invention is addressed to eliminating that reaction without sacrificing the smooth surface which characterizes plaster molds in general. A particular object is to prevent or substantially eliminate any reaction of easily oxidizible metals with permeable plaster molds Without the substantial loss of permeability and accuracy of dimensions. A further object is to provide a permeable plaster mold containing reaction inhibitors which can be readily dried in an oven or in air without formation of an undesirable crust on the mold cavity surface. Still another object is to provide a plaster mix which can be handled in the same manner as conventional plasters which contain no inhibitors. Other objects and advantages of my invention will become apparent from the following description and examples.

I have discovered that by incorporating small amounts of reaction-inhibiting materials and urea formaldehyde resin in a plaster mold a substantially non-reactive smooth surfaced article is produced. The invention finds special application in permeable plaster molds. In the production of such molds the inhibitors and resin are incorporated in the plaster slurry, the slurry made into a foam and molded in conventional manner, or a non-foamed slurry may be molded and the products subjected to the well known recrystallization and rehydration process. In particular, I have found that by means of my invention castings of magnesium and magnesium base alloys can be easily and successfully made which possess a highly satisfactory surface and a fidelity of detail such as has characterized castings of other metals made in plaster molds. In referring to molds it is to be understood that this term includes not only an entire mold but parts of molds such as cores, risers, or other sections which may be assembled 2,753,608 Patented July 10, 1956 with other plaster parts or parts made of other mold materials such as sand or metal.

The base material used in the production of the molds is the familiar calcined gypsum, CaSO4. /zHzO, which when fully hydrated yields the product CaSO42H2O. Although the kettle boiled type of calcined gypsum can be employed .I prefer to use that variety known in the trade as alpha gypsum, such as produced in accordance with teachings of U. 8. Patent 1,901,051. Alpha gypsum offers the advantages of developing a greater strength and density and requiring less water to produce a normal pouring consistency than the kettle calcined material. For the purpose of diluting the calcined gypsum or developing particular properties in a mold or mold part it is a well known practice to mix filler materials with the gypsum. Such substances as talc, asbestos and silica are examples of conventional fillers. In general, not more than 50% by Weight of the plaster mix should be composed of fillers and usually only a few percent, less than 5%, is sufficient. in addition to the fillers, it is frequently desirable to include accelerators or retarders of setting of the calcined gypsum such as sodium sulphate, sodium chloride, sodium citrate and citric acid. These substances are commonly used in amounts of less than 1% by weight of the plaster mix.

The desired permeability of the plaster molds may be developed in different ways. The plaster slurry may be aerated and then allowed to set or the slurry may be poured without aeration and subsequently recrystallized and rehydrated. The latter process is that commonly referred to as the Antioch process which is described in U. S. Patent 2,220,703. Where the slurry is to be aerated a small quantity of foaming agent should be incorporated in the mix which permits the production of bubbles that remain intact until the plaster sets when their walls rupture to some extent with the result that passageways are provided in the mold for escape of mold gases. Sulphomated hydrocarbons, as a class, are suitable foaming agents but of this group the sodium salts of alkyl aryl sulphonates are preferred. From about 0.1 to 1% by Weight of these materials is generally sufficient to supply the desired foaming characteristics.

As mentioned hereinabove the calcined gypsum combines with water to form the fully hydrated calcium sulphate in the mold. In addition to the water of hydration there may be a small amount of free or uncombined Water present in the final product depending upon the thoroughness of the drying operation. It has been found that the presence of as much as 5% by weight of free Water in the permeable mold does not destroy its utility and may even be advantageous under some circumstances. The water may react with the easily oxidizable metal but this tendency is suppressed by the addition of the inhibitors referred to hereinbelow.

To overcome or substantially reduce the reaction be tween easily oxidizible metals, particularly magnesium and magnesium base alloys, and the permeable plaster molds it has been found to be necessary to include one or more of the following substances in the mold composition: boric acid, alkali fluorides, alkali borofluorides and alkali silicofiuoridcs. Sulphur may also be advantageously employed in combination with one or more of the foregoing compounds. The amount and choice of inhibitors will vary with the thickness of the mold section and the mass of metal held in the mold. Obviously, where molten metal remains in contact with the mold for a relatively long period of time it is necessary to provide more protection against reaction than where freezing occurs very quickly. Moreover, better results are generally obtained by using two or more inhibitors, such as boric acid and alkali borofluoride, or boric acid and an alkaline silicothis result.

fluoride or an alkali borofluoride and sulphur. A combination which has been particularly effective is one composed of boric acid, sulphur and an alkali borofluoride. In referring to alkali compounds it is to be understood that this term includes ammonium compounds as well as those of the alkali metals. For most purposes from 0.5 to 5% by weight of the inhibitor is sufficient and where two or more are employed the total amount should not exceed about 8%.

Examples of combinations of inhibitors and their proportion by weight with respect to the dry plaster are as follows: 3% KBF4, 1% boric acid and 1% sulphur; 1% KBF4, /2% boric acid and 1% sulphur; 1% boric acid and 1% sulphur.

The group of inhibitors named above, with the excep- :tion of sulphur, tend to migrate to the surface of the plaster mold in varying degrees as the mold is dried preparatory to its use, apparently because of their slubility in the water present in the mold. They are actually soluble in water to some extent although the proportion is very small in some cases. The migration develops a concentration of the inhibitors on the mold surface which results in a crust when the water is evaporated.

is therefore associated with compounds which do migrate.

In any event, it is not to be included in the class of inhibitors defined above which give rise to the objectionable crust formation.

The process wherein a water soluble substance migrates to an exposed surface and the water evaporates thereby leaving a residue is known as effiorescence. In the case of the plaster molds efilorescence may occur over the entire mold surface or only in certain areas but this is immaterial 'as long as it would result in the production of defective castings.

I have found that the foregoing tendency of reaction inhibitors to effloresce can be very substantially reduced or completely eliminated by also incorporating urea formaldehyde resin in the plaster mix. The presence of the resin in some manner restrains the migration of the inhibitors or forms a barrier through which they cannot readily pass to reach the mold surface. Regardless of .the mechanism by which the resin suppresses efilorescence I have determined that it is very effective in accomplishing I have also ascertained that the addition of the resin and reaction inhibitors can be made without substantially changing the strength and permeability of the plaster mold. This is of great practical importance since it means that the design of a mold need not be changed where a reactive metal is being poured.

The urea formaldehyde resin to be used may be in either solid or liquid form and, if course, is in a partially condensed condition. The drying of the mold and exposure to the air serves to complete the condensation of the resin. If the resin is to be used in solid form then it should be in a condition which will permit solution or dispersion in water. Those forms of urea formaldehyde which are sold in the trade as core binders have been found to be very satisfactory for purposes of my invention.

The amount of resin required to suppress formation of the undesirable crust will vary with the amount of the inhibitors and the thickness of the mold Wall, but generally from 0.2 to by weight of the dry plaster is suflicient for this purpose. In my preferred practice from about 0.7 to 3% by weight of the resin should be employed. In cases where the resin is in liquid form the proportion should be based upon the resin content of the liquid.

In preparing the slurry from which the plaster molds are made the reaction inhibitors, as well as the resin components, may be added to plaster in dry form or as a solution or dispersion. Other components of the mixture, of course may be added in any convenient form. It is often more convenient to dissolve the soluble or dispersable materials in the water used to form the slurry and thus quickly obtain uniform distribution throughout the plaster mix.

To obtain a good pouring consistency in the slurry the Water or the solution containing certain components should be added to the dry plaster in the proportion of about 8 to 11 parts of the former to 8 to 11 parts of the latter. The proportions will vary somewhat according to the composition of the mold mixture but generally the proportions necessary to provide a satisfactory pouring consistency will be obtained by observing the foregoing proportions.

Where the slurry is to be aerated before being poured it is stirred with a suitable device which serves to introduce air and create small bubbles and to uniformly distribute them throughout the slurry. If desired, a foam may be separately produced with a water and foaming agent and the foam added to the dry plaster mix. In any event the volume of the slurry should be substantially increased by generation of the foamed condition. To obtain the desired permeability in the final product it is advisable to increase the volume of the slurry by at least 75% and in many cases by as much as 150 to 200%.

The addition of urea formaldehyde resin, it has been learned, can reduce the permeability of the mold if proper precautions are not followed. Such a reduction may be permissible in some cases if it does not go below the minimum found to be necessary to secure good metal castings. To insure adequate permeability, however, it is desirable to exceed the value that would normally be satisfactory in the absence of the resin and the reaction inhibitor. In the case of molds produced from aerated slurries it is well to increase the volume of the slurry about 150% as compared to a increase which might be otherwise satisfactory. The additional aeration requires but a slightly longer period of agitation and consequently it does not add significantly to the cost of the mold.

As mentioned hereinabove the invention finds particular application in the production of magnesium and magnesium base alloy castings. Other metals and alloys which are easily oxidizible, such as aluminum base alloys containing substantial amounts of magnesium, and reactive toward gypsum may be satisfactorily cast in the same type of mold. In the case of magnesium and magnesium base alloys it is sometimes helpful to add from 0.0001 to 0.05% by weight of berryllium to improve their resistance to oxidation.

The efflorescent tendency of some reaction inhibitors in varying proportions and combinations thereof in plaster articles made with alpha gypsum are shown in the examples below. For these tests cylinders 1%" in diameter and 4" in length were prepared from aerated slurries containing a foaming agent. The reaction inhibitors were added to the dry plaster either as water solutions or in dry form. The proportions were based upon the weight of the dry plaster. The slurry in each case was produced by using 9 parts of water containing a small amount of foaming agent, with or without the reaction inhibitor, and 10 parts of alpha gypsum. The slurries were aerated to the extent of producing an increase in volume of 100 to After being poured the test cylinders were dried in air for two days and then further dried for two hours at 180 F. The test results. appear: in Table; I below:

It is. evident that the migration; and. effiorescent tendency of various. inhibitors does vary and, that the amount may affect the quantity of efiiorescent product in some instances.

The effect of urea formaldehyde. resin upon the efllorescent tendency, the permeability and strength of 2" diameter plaster cylinders similar to those described above is illustrated in the following examples. The resin, initially in dry form, was dispersed in the. water employed to form the plaster slurry while the reaction inhibitors were incorporated in the dry plaster mix before. making the slurry. The slurry was prepared by adding one part of the solution containing the resin to one part of the dry plaster and the slurry was. aerated to produce an increase in volume of 150%. The cylinders were air dried for two days and finally heated for two hours at 190 F. to complete the drying. The test results are set forth in Table II below:

TABLE II Effect of resin upon ejfiorescence Resin AFS Compres- Inblbitor Addition Efliorescenee Permeave percent billty Strength,

. p. s. i.

None 150 90 0 Severe 62 115 0. 1 Moderate. 162 75 0.3 Very Slight 98 85 O. do 50 120 2 Trace 45-60 200i25 3 None 35-50 225:};25

From the foregoing it is apparent that the addition of the urea formaldehyde resin greatly reduced or eliminated efilorescence. Also, the permeability is within the AFS range of 20 to 200 which is usually specified for green and dry sand cores and molds. The compressive strengths also come within the range which characterizes green and dry sand cores, and molds namely, 50 to 250 p. s. i.

Having thus described my invention and certain embodiments thereof, I claim:

1. The method of inhibiting the reaction between easily oxidizable metals and permeable gypsum plaster molds free from lime while retaining a smooth surface on said molds comprising incorporating in the mixture from which a mold is formed at least one reaction inhibitor of the group consisting of boric acid, alkali fluoride, alkali borofluorides and alkali silicofluorides and 0.2 to 5 per cent by weight of urea formaldehyde resin, forming the mold, drying said mold to the extent that it does not contain more than 5 per cent by weight of free water but does retain the water of hydration in the plaster and thereafter casting easily oxidizable metal in said mold.

2'. The method of inhibiting the reaction between easily oxidizable metals and permeable gypsum plaster molds free from lime while retaining a smooth surface on said molds comprising incorporating in the mixture from which the mold is formed from 0.5 to 5 per cent by weight of at least one reaction inhibitor of the group consisting of boric acid, alkali fluoride, alkali borofiuorides and alkali silicofluorides, the total amount of said inhibitors not exceeding 8 per cent by weight of the dry plaster, and 0.2 to 5 per cent by weight of urea formaldehyde resin, forming the mold, drying said mold to the extent that it does not contain more than 5 per cent by weight of free water but does retain the water of hydration in the plaster and thereafter casting easily oxidizable metal in said mold.

3. The method of inhibiting the reaction between easily oxidizable metals and permeable gypsum plaster molds free from lime while retaining a smooth surface on said molds comprising incorporating in the mixture from which the mold is formed 0.5 to 5 per cent by weight. of sulphur, 0.5 to 5 per cent by weight of at least one reaction inhibitor of the group consisting of boric acid, alkali fluoride, alkali borofiuoride and alkali silicoilucride, the total amount of said sulphur and oxidation inhibitor not exceeding 8 per cent by weight of the dry plaster, and 0.2 to 5 per cent by weight of urea formaldehyde resin, forming the mold, drying said mold to the extent that it does not contain more than 5 per cent by weight of free water but does retain the water of hydration in the plaster and thereafter casting easily oxidizable metal in said mold.

4. The method of inhibiting the reaction between easily oxidizable metals and permeable gypsum plaster molds free from lime while retaining a smooth surface on said molds comprising incorporating in the mixture from which the mold is to be formed a sulfonated hydrocarbon foaming agent, 0.5 to 5 per cent by weight of at least one reaction inhibitor of the group consisting of boric acid, alkali fluoride, alkali borofluoride and alkali silicofluoride, the total amount of said inhibitors not exceeding 8 per cent by weight of the dry plaster, and 0.2 to 5 per cent by weight of urea formaldehyde resin, aerating the plaster mixture, forming the mold, drying said mold to the extent that it does not contain more than 5 per cent by weight of free water but does retain the water of hydration in the plaster and thereafter casting easily oxidizable metal in said permeable mold.

5. The method of inhibiting the reaction between easily oxidizable metals and permeable gypsum plaster molds free from lime while retaining a smooth surface on said molds comprising incorporating in the mixture from which the mold is to be formed 0.5 to 5 per cent by Weight of at least one reaction inhibitor of the group consisting of boric acid, alkali fluoride, alkali borofluoride and alkali silicofiuoride, the total amount of said reaction inhibitors not exceeding 8 per cent by weight of the dry plaster, and 0.2 to 5 per cent by weight of urea formaldehyde resin, forming the mold, recrystallizing and rehydrating said mold, to the extent that it does not contain more than 5 per cent by weight of free water and thereafter casting easily oxidizable metal in said permeable mold.

6. A lime-free permeable gypsum plaster mold substantially non-reactive toward easily oxidizable metal containing fully hydrated gypsum as the principal component, 0.5 to 5 per cent by weight at least one reaction inhibitor of the group consisting of boric acid, alkali fiuoride, alkali borofluoride, and alkali silicofluoride, the total amount of said reaction inhibitors not exceeding 8 per cent by weight of the dry plaster, 0.2 to 'per cent by weight of urea formaldehyde resin and containing less thanS per cent by weight of free water, said mold being characterized by a smooth, substantially noneflloresced surface. I 7. A lime-free permeable gypsum plaster mold substantially non-reactive toward easily oxidizable metals containing fully hydrated gypsum as the principal component, from 0.5 to 5 per cent by weight of sulphur, 0.5 to 5 per cent by weight of at least one reaction inhibitor of the group consisting of boric acid, alkali fluoride, alkali borofluoride and alkali silicofluoride, the total amount of sulphur and said reaction inhibitors not exceeding 8 per cent by weight of the dry plaster, 0.2 to 5 per cent by weight of urea formaldehyde resin and containing less than 5 per cent by weight of free Water, saidmold being characterized by a substantially smooth non-efiloresced surface.

8. A lime-free permeable gypsum plaster mold substantially non-reactive toward easily oxidizable metals containing fully hydrated gypsum as the principal component, 0.5 to 5 per cent by weight of a reaction inhibitor of the group consisting of boric acid, alkali fluoride, alkali borofluoride and alkali silicofluoride, 0.5 to 5 per cent by weight of sulphur, the total amount of said reaction inhibitors and sulphur not exceeding 8 per cent by weight of the dry plaster, 0.2 to 5 per cent by weight of urea formaldehyde resin and containing less than 5 per cent by weight of free waer, said mold being characterized by a permeability and compressive strength equivalent to that of sand cores and molds, and having a substantially smooth non-effloresced surface.

' 9. A lime-free permeable gypsum plaster mold substantially non-reactive toward easily oxidizable metals containing fully hydrated gypsum, up to 50 per cent by weight of filler, 0.5 to 5 per cent by weight of a reaction inhibitor of the group consisting of boric acid, alkali fluoride, alkali borofluoride and alkali silicofluoride, 0.5

to 5 per cent by Weight of sulphur, the total amount of said reaction inhibitor and sulphur not exceeding 8 per cent by weight of the dry plaster, 0.2 to 5 per cent by weight of urea formaldehyde resin and containing 8 lessthan 5 per cent by weight of free-wateflsaid mold being characterized by a permeability and compressive strength equivalent to that of sand cores and molds, and having. a substantially smooth non-effloresced surface.

10. A lime-free permeable gypsum plaster mold substantially non-reactive toward easily oxidizable metals containing fully hydrated gypsum as the principal component, 0.5 to 5 per cent by weight of boric acid, 0.5 to 5 per cent by weight of alkali borofluoride and 0.5 to 5 per cent by weight of sulphur, the total amount of said boric acid, borofluoride and sulphur not exceeding 8 per cent by weight of the dry plaster, 0.2 to 5 per cent by weight of urea formaldehyde resin and containing less than 5 per cent by weight of free water, said mold being characterized by a substantially smooth noncfiioresced surface.

11. A lime-free permeable gypsum plaster mold substantially non-reactive toward easily oxidizable metals containing fully hydrated gypsum as the principal component, 0.5 to 5 per cent by weight of a reaction inhibitor of the group consisting of boric acid, alkali fluoride, alkali borofluoride and alkali silicofluoride, 0.5 to 5 per cent by Weight of sulphur, the total amount of said reaction inhibitor and sulphur not exceeding 8 per cent by weight of the dry plaster, 0.7 to 3 per cent by weight of urea formaldehyde resin and containing less than 5 per cent by weight of free water, said mold being characterized by a substantially smoothmon-efiloresced surface.

References Cited in the file of this patent UNITED STATES PATENTS 1,895,639 Mond Jan. 31, 1933 1,917,759 Gann July 11, 1933 2,148,642 Ricard Feb. 28, 1939 2,152,670 Shutt Apr. 4, 1939 2,220,703 Bean Nov. 5, 1940 2,321,674 Harshberger June 15, 1943 2,362,060 Etridge et a1, Nov. 7, 1944 2,405,650 Hartwig et a1. Aug. 13, 1946 2,422,118 Meyer June 10, 1947 2,529,835 Dailey et al. Nov. 14, 1950 

1. THE METHOD OF INHIBITING THE REACTION BETWEEN EASILY OXIDIZABLE METALS AND PERMEABLE GYPSUM PLASTER MOLDS FREE FROM LIME WHILE RETAINING A SMOOTH SURFACE ON SAID MOLDS COMPRISING INCORPORATING IN THE MIXTURE FROM WHICH A MOLD IS FORMED AT LEAST ONE REACTION INHIBITOR OF THE GROUP CONSISTING OF BORIC ACID, ALKALI FLUORIDE, ALKALI BOROFLUORIDES AND ALKALI SILICOFLUORIDES AND 0.2 TO 5 PER CENT BY WEITHT OF UREA FORMALDEHYDE RESIN, FORMING THE MOLD, DRYING SAID MOLD TO THE EXTENT THAT IT DOES NOT CONTAIN MORE THAN 5 PER CENT BY WEIGHT OF FREE WATER BUT DOES RETAIN THE WATER OF HYDRATION IN THE PLASTER AND THEREAFTER CASTING EASILY OXIDIZABLE METAL IN SAID MOLD. 